1. Field of the Invention
This invention relates generally to semiconductor processing, and more particularly to stacked semiconductor chip devices and methods of making and assembling the same.
2. Description of the Related Art
A typical conventional flip-chip packaged semiconductor chip consists of a laminate of several layers of different materials. From bottom to top, a typical package consists of a base or carrier substrate, a die underfill material, an array of solder joints and the silicon die. For some designs, a thermal interface material and a lid or heat spreader top off the stack. In some designs the carrier substrate includes a ball grid array to connect to another circuit board. Each of the layers of the package generally has a different coefficient of thermal expansion (CTE). In some cases, the coefficients of thermal expansion for two layers, such as the underfill material and the silicon die, may differ by a factor of ten or more. Materials with differing CTE's strain at different rates during thermal cycling.
Some conventional chip packages incorporate stacked dice. Like a conventional package substrate, a conventional die typically includes plural layers of different materials, such as a bulk semiconductor layer, an active device layer, and multiple metallization layers interspersed with dielectrics. Such metallization may be on the front and backside of the die. Not surprisingly, the layers of the conventional die have different CTE's.
Thus for a conventional stacked dice and carrier substrate arrangement, the multitude of different CTE's will tend to produce differential strain rates that can produce warping of the carrier substrate and the silicon dice. If the warping is severe enough, several undesirable things can occur.
One risk associated with stacked dice warping is solder joint delamination. If the warping is severe enough, some of the solder joints between the stacked dice can delaminate or fracture and cause electrical failure.
Another pitfall associated with die warping is the potential difficulty in establishing metallurgical bonds between interconnects of the mating dice. The warping causes the lower surfaces of interconnect structures of one or both the stacked dice to be non-planar. Depending on the direction of warping, the interconnects at the outer edges of one of the dice may be either higher or lower than those near the interior. If a given interconnect of one die is too far away from a corresponding interconnect on the other die at the time of reflow, the interconnects may not merge to form a solder joint after reflow and leave an open circuit.
The present invention is directed to overcoming or reducing the effects of one or more of the foregoing disadvantages.